1. Field of the Invention
The field of the invention is the electrometallization of non-metallic substrates without using an electroless metal coating. In one embodiment, the invention relates to circuit boards and a method for improving the manufacture of such boards by eliminating electroless plating of the boards and effecting through-hole plating and/or directly forming circuits thereon by an electrolytically deposited metal coating. A novel coating process and composition are disclosed for this process.
2. Discussion of the Related Art
Many processes are known for the formation of metal coatings on non-metallic substrates either for decorative or functional purposes. One of the more commercially important uses of such coatings is in the manufacture of printed circuit boards. Printed circuit boards (PCB's) comprise, for example, a rigid non-conducting or dielectric base made up of a fibrous material such as glass fibers, paper and the like in combination with a polymer such as an epoxy resin, and a conductive metal layer such as copper on either one or both surfaces. Multilayer boards (MLB's) comprise several PCB's laminated to one another by means of an adhesive. In addition to rigid boards (as described above), flexible boards can be produced employing thermoplastic dielectric layers such as fluorocarbon polymers, Nylon polymers, polyimides, Kevlar.TM. reinforced polymers, polyparabanic acids and polyesters. Flexible boards are manufactured without fiber reinforcing. Production of both of these types of printed circuit boards are described in Printed Circuits Handbook, Second Edition, edited by C. F. Coombs, Jr., McGraw-Hill, 1979, which is incorporated herein by reference. Laminated combinations of flexible and rigid boards are also finding utility in some applications for MLB's.
In the manufacture of PCB's, a metal conductive foil such as copper is bonded to the circuit board, although any metal may be applied to a non-conductive dielectric circuit board as a foil or by electro deposition or electroless deposition.
Prior to laminating inner layers to form an MLB, the metal surface is treated in an art known manner to produce electrically conductive lines (circuits) for the transfer of current between components of an electric circuit, the components comprising by way of example diodes, transistors, resistors, capacitors and the like. The circuits may be formed either by a positive or a negative working photoresist, silk screen resist or hand painted resist process followed by etching and in some instances, electrodeposition of a metal or metals, all of which is known in the art.
In forming MLB's by laminating, an adhesive in the form of a prepreg is inserted between the surfaces of the PCB's that are to be laminated, after which the multilayers are further treated by application of heat and pressure. The prepreg generally comprises a woven or non-woven layer or layers of fibers such as glass, cellulose (e.g., paper), and the like, glass being preferred. The prepreg also is impregnated with a so-called "B-stage" resin such as an epoxy resin that has been partially cured. Art known equivalents of epoxy resins are also employed as adhesives such as acrylic resins (used with polyimide circuit boards) or polyester resins.
In MLB's, the circuit of one board is connected to the circuit of one or more of the other boards in the multilayers. This is done by forming pads or circular areas of metal at a point or points on the conductive line or lines of the board. The pads may also be isolated from the conductive lines. The other board or boards that are to be connected are similarly provided with pads and in the laminating process the pads of the different boards are aligned over one another.
The MLB is then pressed and cured after which the pads of the inner layers are drilled to form through-holes. The diameter of the drill is considerably less than the diameter of the pad, the ratio of diameters of the pad to the drill being about 2:1 or greater, so that the overall structure comprises at a minimum, a pad from one board aligned over a pad from another board, with a through-hole passing through them. Since the through-hole in cross-section ideally presents a surface of alternating layers of the pads of the individual PCB's separated by the non-conductive base, an electrically conductive element has to be employed in the hole to form an electrical connection between the pads. This is done by a process known in the art as through-hole plating (PTH).
PTH processes are also employed for connecting two metal conductive surfaces having a single non-conductive or dielectric board interposed therebetween for the formation of a PCB. Double sided boards of this type and the formation of through-holes in such boards are to be considered as falling within the scope of the present invention and are intended to be included within the broad definition of a PCB as that term is used throughout the written description of the present specification.
Before the PTH process can be undertaken, any "smear" in the hole must be removed. Smearing is encountered when the drill bit employed to form the hole through the aligned pads in an MLB picks up resinous material from the hole and deposits this material on the wall of the hole during the drilling process. Since the wall of the hole contains alternating resinous material layers and metal layers, the surface of the metal layers that form part of the hole wall will be coated with the resinous material thereby preventing any metallic plating material applied to the surface of the hole wall from contacting the metal layers and forming an electrically conductive connection with it. It is believed that the resinous material such as a B-stage epoxy resin used in the prepreg comprises the principle material involved in the smearing of the hole. Smearing, therefore, renders the PTH process ineffective.
The problem of smearing is overcome by chemical cleaning in which a strong mineral acid such as sulfuric acid (optionally followed by hydrofluoric acid) is used in an "etch-back" process to away the "smear" from the wall of the hole. Hydrofluoric acid is used to remove any glass fiber ends that might be projecting into the hole. The glass fiber ends come from the glass fiber employed in the manufacture of the circuit boards or prepreg and are removed since they cause imperfections in the surface of the metallic coating applied by the PTH process and can cause conductivity failures in the hole.
The etch-back process requires very careful control in order to prevent excessive etching of the wall of the hole. The concentration and temperature of the etching solution has to be monitored as well as the length of time over which the etching process is conducted.
After smear is removed, the through-hole is plated. Prior art methods comprise the application of electroless copper as a PTH plating material. Standard electroless copper plating solutions known in the art are used for this purpose. Prior to applying the electroless copper, and in order to promote its deposition on the non-conductive surface, the non-conductive surface is treated with a two step activator comprising a stannous chloride sensitizer solution followed by a sensitizer solution of divalent palladium chloride according to the process of Bergstrom et al., U.S. Pat. No. 2,702,253. The stannous chloride is oxidized to stannic chloride and the palladium chloride reduced to palladium metal on the uncoated portions of the board. The palladium, however, when in contact with the copper cladding of the circuit board, forms a palladium immersion coating on the copper since palladium is more noble than copper and displaces it. This can result in an inordinate consumption of palladium in the process.
A more preferred method for preparing the activator is described by Shipley, Jr., U.S. Pat. No. 3,011,920 which employs an activator comprising a noble metal dispersion e.g., a colloidal palladium dispersion, containing stannic tin. Stannic and/or stannous tin forms a protective colloid around the metallic palladium, and the dispersion implants a precious metal site on the non-conductive surface for the purpose of initiating the deposition of the copper by chemical reduction. This process substantially eliminates the problems of forming immersion palladium coatings on the copper surface of the copper-clad boards. A post activator is then employed, generally an acid, to solubilize the protective colloid and expose the noble metal, i.e., palladium.
The subsequently applied electroless copper coating solution contains cupric ions and a reducing agent such as formaldehyde, which reduces the cupric ions in the solution to copper metal when in the presence of palladium, which acts as a catalyst for the reduction. The copper metal plates out on the surface of the through-hole, making electrical contact with the walls of the metal pads through which the hole is drilled as well as the copper surface on the outer and inner layer(s) of the MLB. The electroless copper may have subsequent metal coatings applied to it by electrolytic means.
The stability and plating characteristics of electroless copper are controlled to a large degree by additives known collectively as stabilizers or rate controllers. Because these additives require a fine balance in the plating bath under various production conditions, the electroless copper process is difficult to control precisely so that consistent results are obtained. This balance is obtained by conducting regular analyses of the components of the electroless bath as well as continuous or periodic addition of the various additives. Temperature control of the bath is also required as well as continuous filtration and aeration. The plating tank and bath filter have to be cleaned regularly and regular plating rate determinations have to be made as well.
Significantly, environmental problems have to be addressed when employing electroless copper, such as removal of the complexing agents employed in the bath from waste-water streams, removal of the metal from the bath prior to disposal of the spent bath, monitoring COD levels in waste-water, reducing such levels and lastly, exposure of operators and the environment to formaldehyde which is a carcinogen. The latter is an especially significant problem.
Various processes and compositions have been developed to avoid the use of electroless copper plating for many of the above reasons. Elimination of the electroless copper coating could also amount to significant cost savings in the production of metallized non-conductive materials such as circuit boards.
One of the systems used to avoid electroless coatings was based on conductive ink technology which relies on the physical deposition of a fine layer of carbon in the through-holes and over the entire board surface.
However, carbon deposits on the metal surface of a circuit board interfere with the adhesion of copper or other metals that are subsequently electroplated onto the base copper and accordingly, the carbon had to be removed by an etching step prior to electroplating. It was difficult to control this etching step and the carbon in the through-holes also had to be protected. For these reasons, the carbon process had only limited commercial acceptance.
Another solution to the problem was the employment of conductive polymers, whereby the circuit board or other non-metallic substrate is immersed in a strong hot permanganate solution to produce manganese dioxide on the substrate, such as the epoxy surfaces of a MLB. When the board thus treated is contacted with a suitable monomer, the conductive polymer is formed on those areas on which the manganese dioxide is formed. One of the advantages of employing this process is there is little conductive polymer deposited on the copper surface of the circuit board. Some difficulties are presented, however, in that glass or other resins such as polyimides and polytetrafluoroethylene are not readily coated by this process. In addition, there are problems encountered in controlling polymer layer thicknesses and monomer instability, as well as some environmental problems. The monomers are generally hazardous substances and, because of their volatility, will escape into the atmosphere and polymerize to form a black film on other areas of the circuit board and the coating equipment.
Radovsky et al., U.S. Pat. No. 3,099,608, assigned to IBM, discloses a process in the early 1960's for the direct metallization of non-conductive non-metallic substrates by a system using a "conductivator" metal such as palladium in colloidal or semi-colloidal form. By very carefully controlling the process, Radovsky et al. found that it was possible to build enough potential across the through-hole portion of a two-sided board to induce copper deposition. Although the process proved to be of little commercial value, several subsequent processes were based on the Radovsky et al. discovery.
For example, Holtzman et al., U.S. Pat. No. 4,891,069 discovered that if the palladium colloid was stabilized with a water soluble salt such as aluminum chloride, the palladium would act to combine with hydrogen that was generated electrolytically and this hydrogen could be employed to reduce a subsequently or simultaneously applied aqueous solution of a copper salt thereby eliminating electroless copper.
At about the same time, Morrissey et al., U.S. Pat. No. 4,683,036 developed the "EE-1" system in which the electroless coating process was also eliminated. In the EE-1 system, palladium "islands" are formed in the through-holes and plated by a special copper bath that contains an inhibitor which generally can be described as a surfactant, chelating agent, brightener or levelling agent.
Although the EE-1 process has some promise as a commercially viable method, it is not especially suitable in pattern plate applications since panel plating is required while the panels are still wet from the catalyzing step. Additionally, the EE-1 process is not especially suitable for effectively plating high aspect ratio multi-layer boards.
Several so-called sulfide conversion coatings can also be employed to electroplate non-conductive substrates without the application of an electroless metal coating such as those described by Bladon, U.S. Pat. Nos. 4,895,739 and 4,919,768, in which a catalytic metal chalcogenide is formed on a non-metallic substrate by applying a tin-noble metal electroless catalyst to the substrate and subsequently treating the substrate with a solution containing a dissolved sulfide to form a sulfide of the noble metal. An electrolytic metal coating is then applied to the substrate.
Gulla et al., U.S. Pat. No. 4,810,333 also describes the application of a transition metal sulfide to a non-metallic substrate adjacent to and in contact with conductive areas on the substrate after which electrolytic plating can be conducted over the sulfide formed. A permanganate solution is given as an example of one of the transition metal compounds that can be employed in the process and is used to form a manganese oxide coating on the substrate. This manganese coating is subsequently converted to a sulfide by means of a metal thiocarbamate solution.
The conductivity of the sulfide conversion coating, as well as the conductive polymers described previously is generally low. Accordingly, it is difficult to avoid what is known in the art as "dog-boning" as plating builds up near the through-hole entrances and in the case of small holes, closing the hole before plating has reached through the center of the hole. Long plating times are also encountered employing either one of these systems and this can contribute to excessive "pink-ring" formation.
Okabayashi, U.S. Pat. No. 5,071,517, assigned to Solution Technology Systems, describes a method for the direct electroplating of a non-conducting substrate where the substrate is first treated with a non-acidic aqueous salt solution containing a micro-fine colloidal dispersion of a noble or precious metal and tin to form a strongly adsorbed, uniform conducting layer upon at least a portion of the substrate. The conducting layer is then electroplated directly. Okabayashi describes the use of an aldehyde such as lignin vanillin and notes that it is utilized to form the micro-fine palladium/tin catalyst. According to the invention, electroless plating systems, conversion coatings or preferential plating solution additives are avoided.
Harnden, in a paper presented at Northeast Circuits Association Technical Conference, Mar. 19, 1992, further describes the Solution Technology Systems catalyst as being easier to process control than electroless copper, noting that the uniformity and fineness of the catalysts are augmented by the use of a food grade aldehyde. Harnden goes on to state that with normal processing, the adsorbed catalyst does not provide enough conductivity to allow processing small holes or high aspect ratios, but by using a special alkaline accelerator, a continuous and highly conductive catalyst film is produced which is easily removed from the copper surfaces of a circuit board by micro-etching. It is further noted by the author that in order to achieve optimal conductivity after the boards are immersed in a warm solution of the tin-palladium colloid catalyst, they are immersed in a mild alkaline bath which also contains a small amount of copper ions which deposit on and between the tin-palladium particles.
Kitaev et al., U.S. Pat. No. 3,984,290 describes a PTH process in which a film is formed by immersing a MLB into a solution containing ions of a metal which is more electropositive than the metal of the metal layers. The film obtained has a monolithic structure in the dielectric zones of the through-hole and a porous structure in the metal zones. Examples of solutions containing ions of a metal which is more electropositive than the metal of the MLB metal layers include silver salt/ammonium hydroxide solutions; palladium copper-salt/sulfuric acid solutions and palladium salt/hydrochloric acid solutions. Prior to immersion of the MLB into one of these solutions, the structure is preferably treated with a conventional sensitizer such as those containing ions of divalent tin or trivalent titanium.